Mastering PHP 7

Nullable types

Many programming languages allow some sort of optional or nullable types, depending on terminology. The PHP dynamic type already supports this notion via the built-in null type. A variable is considered to be of the null type if it has been assigned a constant value null, it has not been assigned any value, or it has been unset using the unset() construct. Aside from variables, the null type can also be used against the function parameters, by assigning them a default value of null.

However, this imposed a certain limitation, as we could not declare a parameter that might be null without flagging it as optional at the same time.

PHP 7.1 addressed this limitation by adding a leading question mark symbol (?) to indicate that ...